Contents
- Features of Waves
- The Wave Equation
- Transverse & Longitudinal Waves
- Wave Behaviour
- Ripple Tank
The diagram below shows a toy duck bobbing up and down on top of the surface of some water, as waves pass it underneath.
Explain how the toy duck demonstrates that waves do not transfer matter.
Answer:
There is a key distinction between the particles (or oscillations) of a wave, and the wave itself. The motion of the wave causes the particles to move. The particles themselves are not the wave.
Waves can be shown through vibrations in ropes or springs.
Wave motion of water waves may be demonstrated using a ripple tank.
When describing wave motion, there are several terms which are important to know, including:
Small water waves are created in a ripple tank by a wooden bar. The wooden bar vibrates up and down hitting the surface of the water. The diagram below shows a cross-section of the ripple tank and water.
Identify the letter which shows:
a) the amplitude of a water wave.
b) the wavelength of the water wave.
Answer:
Part (a)
[Diagram: The same ripple tank cross-section, but with a horizontal line drawn through the middle of the waves, labeled "UNDISTURBED POSITION".]
Part (b)
The equation used to calculate wave speed is:
v = f × λ
Where:
The period of a wave is defined as: The time taken for one complete oscillation to pass a fixed point.
The frequency of a wave is related to its period by the following equation:
f = 1 / T
And therefore,
T = 1 / f
Where:
A wave in a pond has a speed of 0.15 m/s and a time period of 2 seconds. Calculate:
a) The frequency of the wave
b) The wavelength of the wave
Answer:
Part (a)
Part (b)
When stating equations make sure you use the right letters. For example, use λ for wavelength, not L or W. If you can't remember the correct letters, then just state the word equations. Be careful with units: wavelength is usually measured in metres and speed in m/s, but if the wavelength is given in cm you might have to provide the speed in cm/s. Likewise, watch out for the frequency given in kHz: 1 kHz = 1000 Hz.
Transverse waves can be seen in a rope when it is moved quickly up and down. Examples of waves that can be modelled as transverse are:
Longitudinal waves can be seen in a slinky spring when it is moved quickly backwards and forwards. Examples of waves that can be modelled as longitudinal waves are:
| Property | Transverse waves | Longitudinal waves |
|---|---|---|
| Structure | Peaks and troughs | Compressions and rarefactions |
| Vibration | Right angles to the direction of energy transfer | Parallel to the direction of energy transfer |
| Vacuum | Only electromagnetic waves can travel in a vacuum | Cannot travel in a vacuum |
| Material | Can move in solids and the surfaces of liquids | Can move in solids, liquids and gases |
| Density | A constant density | The density of the wave changes |
| Pressure | Has a constant pressure | Pressure in the wave changes |
| Speed of wave | Depends on the material the wave is travelling in | Depends on the material the wave is travelling in |
The key difference between transverse and longitudinal waves is the direction of the vibrations with respect to the direction of the wave itself. For transverse waves, these are perpendicular to each other, whilst for longitudinal waves, these are parallel.
All waves, whether transverse or longitudinal, can undergo:
An identical image of the tree is seen in the water due to reflection.
Diffraction: when a wave passes through a narrow gap, it spreads out.
When drawing waves being reflected take care to:
Similarly, when waves are diffracted the wavelength remains constant. Refraction is the only wave effect in which the wavelength changes. Remember: Refraction is the name given to the change in the speed of a wave when it passes from one medium to another. The change in direction is a consequence of this.
The size of the gap (compared to the wavelength) affects how much the waves spread out.
When a wave goes past the edge of a barrier, the waves can curve around it. Shorter wavelengths undergo less diffraction than longer wavelengths.
Ripple tanks are commonly used in experiments to demonstrate the following properties of water waves:
Incident wavefronts are reflected at 90 degrees against a barrier.
Wavefronts of incident and reflected waves form right angles to each other.
When water waves travel from deep areas to shallow areas they slow down.
When the gap size is bigger than the wavelength less diffraction occurs and the waves spread out less after passing through.
Ripple tank patterns for low and high-frequency vibration.